In present optical communication system design, a method of preliminary design is employed in which design is carried out before communication services are offered based on information on system conditions that are given in advance, for example, fiber parameter, signal modulation format, bit rate, fiber input light power, or the like. In this case, it is difficult to measure actual parameters and therefore normally values defined in consideration of a margin of specifications are used for the design.
However, it is difficult to estimate an appropriate margin. Depending on the case, there is a problem that, although the actual performance is better, worse design values than the actual performance are output and the transmittable distance becomes shorter due to setting of an excessive margin. In recent years, requests for the optical communication system to have longer distance and higher capacity have been strong and such a problem becomes an issue.
Thus, there is the following technique. While the signal-to-noise ratio (SNR) is adjusted on the generating side of signal light, the output light thereof is transmitted by an optical transmission line. Then, a quality indicator based on the SNR and the bit error rate (BER) of the signal light output from the optical transmission line is measured and the degree of margin in normal operation is measured (for example, Japanese Laid-open Patent Publication No. 11-230857).
Moreover, there is a technique in which the optimum route is selected based on a total optical signal-to-noise ratio (OSNR) index value calculated for each of routes from an optical node at the starting point to an optical node at the ending point by using an OSNR index value calculated based on the OSNR of the transmission paths between the respective optical nodes (for example, Japanese Laid-open Patent Publication No. 2007-82086).
Here, the SNR or OSNR is the optical signal to noise ratio. The BER is the bit error rate.
According to the above-described related art, the degree of margin in normal operation may be calculated based on the measurement result of the SNR when the BER is somewhat high. However, it is difficult to accurately estimate the SNR when the BER in normal operation is low directly because the measurement error in a line card that is an optical transmitter/receiver is large.
Also in the related art in which the OSNR in the whole route is calculated based on the OSNR of the transmission paths between the respective optical nodes, the measurement error is large again regarding the OSNR when the BER of the transmission path between the optical nodes is low. Thus, the OSNR in the whole route is also difficult to accurately estimate. In view of the above, it is desirable that the transmission quality may be accurately measured irrespective of variation in characteristics of the optical transmission device.